1. Field of the Invention
The present invention relates to a process for preparing a hydrophobic precipitated silica featuring extremely low water absorption, a high level of whiteness, and properties of low thickening in silicone rubber formulations with a high reinforcing action in the silicone vulcanizates.
2. Description of the Background
The treatment of finely divided solids, metal oxides, and silicates with organosilicon compounds, such as organopolysiloxanes, is known, for example, as disclosed in DE 30 85 905. The process involves a heat treatment process which is conducted under an inert atmosphere of nitrogen. Additionally, hydrophobic silicates are known, for example, as described in DE 24 35 860, DE 22 42 728, and DE 25 13 608.
In these documents, hydrophilic silicates and precipitated silicas are hydrophobicized by reacting them with organosilicon compounds. Examples of hydrophobicizers used include organohalosilanes and organopolysiloxane compounds.
DE 26 28 975 and DE 27 29 244 describe preparing hydrophobic silicas by reacting a hydrophilic precipitated silica featuring low water absorbency with silicone oil or dimethyldichlorosilane, respectively. In the process described in DE 26 28 975, the reaction is conducted with the hydrophobicizer (silicone oil) being added to the dry precipitated silica. In the process described in DE 27 29 244, the hydrophobicizer (dimethyldichlorosilane) is introduced directly into the precipitated silica suspension. In both cases, the hydrophobicizing step is followed by heat treatment at elevated temperatures, specifically ranging from 200 to 400° C.
A disadvantage of this process is that the precipitated silica thus hydrophobicized becomes discolored at the required process temperatures. The discoloration of this silica is particularly inconvenient when it is added to silicone formulations; that is, when these hydrophobic precipitated silicas are added to silicone rubber formulations or to defoamer mixtures based on silicone oil.
As a measure of the discoloration it is possible to employ the value known as reflectance. In measuring the reflectance of hydrophobic precipitated silica, the diffuse reflection power of a sample is investigated. The higher the diffuse reflection power of a sample, the higher its reflectance and thus the higher the whiteness of the sample.
Precipitated silicas generally have a reflectance of not more than 97%. (In this regard, an object of the present invention is to hydrophobicize silicas in such a way that, ideally, the reflectance of the original silica is retained.) Discoloration occurs, in particular, when the precipitated silicas are strongly hydrophobicized; that is, have a high methanol wettability and high carbon loadings It is precisely these properties, however, which are in many cases (in silicone rubber formulations, for example) desired.
A further disadvantage of the known processes is that only a limited amount of hydrophobicizer can be attached covalently to the silica. Particularly in silicone rubber formulations, however, high carbon loadings are desired, since they permit decisive improvements in the rheological properties, such as the thickening, i.e., low yield point and low viscosity, of the compounds.
As a measure of the thickening of the treated silica, it is convenient to use what is termed the DBP number. The DBP number indicates the absorption capacity of a silica for DBP. The measurement technique shows the amount of dibutyl phthalate, in grams, on a sample of 100 g, at which a massive increase in force in the compounder is observed.
It is also not possible to achieve high carbon loadings by using diorganodichlorosilanes, or hydrolysis products of diorganodichlorosilanes, or with corresponding diorganopolysiloxanes in excess to the silanol groups present, because the totality of the organosilicon compounds is no longer attached covalently to the silica. In hydrophobicized silicas for fractions of hydrophobicizing agent that have not been covalently attached, there is a risk that these molecules may have a marked mobility, which in many applications can be very detrimental (e.g., in silicone rubber applications for medical purposes or for articles that are safe in food contact, such as pacifiers, etc.).
A further disadvantage of the prior art processes is that the relatively low carbon contents of less than 3.1% lead to hydrophobic silicas which have a strong thickening action in silicone rubber formulations. DE 26 28 975 lists data on the testing of hydrophobic precipitated silica in silicone rubber formulations, in which the hydrophobic precipitated silica is used in increasing weight fractions. From these data it is clear that, at a level of just 15% of hydrophobic silica in the rubber, the self-leveling properties of the silica disappear and that, at 20%, flowable compounds are no longer obtained. All tables clearly indicate that all of the mechanical properties are improved as the filler content goes up. It would therefore be desirable to prepare silicone rubber formulations which include high fractions of hydrophobic silicas, for improving the mechanical properties, but which at the same time are still flowable.